该客户数据库在春节之前就出现故障，后面经过多人尝试恢复后，均为打开数据库；数据库在open时报如下
Wed Jan 13 17:03:25 2016
ORA-01555 caused by SQL statement below (SQL ID: 4krwuz0ctqxdt, Query Duration=1452675805 sec, SCN: 0x0d6a.46c6524f):
Wed Jan 13 17:03:25 2016
select ctime, mtime, stime from obj$ where obj# = :1
Wed Jan 13 17:03:25 2016
Errors in file /u01/app/oracle/admin/xxxx/udump/xxxx1_ora_18274.trc:
ORA-00704: bootstrap process failure
ORA-00704: bootstrap process failure
ORA-00604: error occurred at recursive SQL level 1
ORA-01555: snapshot too old: rollback segment number 20 with name "_SYSSMU20$" too small
Error 704 happened during db open, shutting down database
USER: terminating instance due to error 704
Instance terminated by USER, pid = 18274
ORA-1092 signalled during: alter database open resetlogs...
Wed Jan 13 17:06:34 2016
该错误其实很场景，也恢复过太多这种情况了，这里不再过多描述。不过这里让我感觉很诧异的是该SQL的Query Duration 太大了。
根据经验，这种情况下可以直接推进SCN。可是当我们进行如下操作，发现不起作用：
alter session set events ’10015 trace name adjust_scn level 13740′;
进一步通过10046 trace跟踪发现该sql访问了如下几个block：
PARSING IN CURSOR #5 len=52 dep=1 uid=0 oct=3 lid=0 tim=1422682994194207 hv=429618617 ad='395fa870'
select ctime, mtime, stime from obj$ where obj# = :1
END OF STMT
PARSE #5:c=0,e=225,p=0,cr=0,cu=0,mis=1,r=0,dep=1,og=4,tim=1422682994194205
BINDS #5:
kkscoacd
Bind#0
oacdty=02 mxl=22(22) mxlc=00 mal=00 scl=00 pre=00
oacflg=08 fl2=0001 frm=00 csi=00 siz=24 off=0
kxsbbbfp=2b7694a4aea8 bln=22 avl=02 flg=05
value=20
EXEC #5:c=0,e=398,p=0,cr=0,cu=0,mis=1,r=0,dep=1,og=4,tim=1422682994194653
WAIT #5: nam='db file sequential read' ela= 20378 file#=1 block#=218 blocks=1 obj#=-1 tim=1422682994215120
WAIT #5: nam='db file sequential read' ela= 480 file#=1 block#=219 blocks=1 obj#=-1 tim=1422682994215712
WAIT #5: nam='db file sequential read' ela= 18990 file#=1 block#=122 blocks=1 obj#=-1 tim=1422682994234841
。。。。。。。
EXEC #6:c=0,e=141,p=0,cr=0,cu=0,mis=0,r=0,dep=2,og=3,tim=1422682994267351
FETCH #6:c=0,e=34,p=0,cr=2,cu=0,mis=0,r=1,dep=2,og=3,tim=1422682994267413
STAT #6 id=1 cnt=1 pid=0 pos=1 obj=15 op='TABLE ACCESS BY INDEX ROWID UNDO$ (cr=2 pr=0 pw=0 time=28 us)'
STAT #6 id=2 cnt=1 pid=1 pos=1 obj=34 op='INDEX UNIQUE SCAN I_UNDO1 (cr=1 pr=0 pw=0 time=16 us)'
WAIT #5: nam='db file sequential read' ela= 12312 file#=7 block#=4993 blocks=1 obj#=-1 tim=1422682994279882
WAIT #5: nam='db file sequential read' ela= 18776 file#=7 block#=4965 blocks=1 obj#=-1 tim=1422682994298789
WAIT #5: nam='db file sequential read' ela= 13157 file#=7 block#=4801 blocks=1 obj#=-1 tim=1422682994312081
WAIT #5: nam='db file sequential read' ela= 12519 file#=7 block#=4954 blocks=1 obj#=-1 tim=1422682994324726
WAIT #5: nam='db file sequential read' ela= 410 file#=7 block#=4952 blocks=1 obj#=-1 tim=1422682994325259
WAIT #5: nam='db file sequential read' ela= 5447 file#=7 block#=4778 blocks=1 obj#=-1 tim=1422682994330830
WAIT #5: nam='db file sequential read' ela= 12349 file#=7 block#=5184 blocks=1 obj#=-1 tim=1422682994343291
WAIT #5: nam='db file sequential read' ela= 11874 file#=5 block#=8645 blocks=1 obj#=-1 tim=1422682994355283
WAIT #5: nam='db file sequential read' ela= 4925 file#=5 block#=8595 blocks=1 obj#=-1 tim=1422682994360323
FETCH #5:c=4999,e=165865,p=15,cr=18,cu=0,mis=0,r=0,dep=1,og=4,tim=1422682994360535
ORA-00704: bootstrap process failure
ORA-00704: bootstrap process failure
ORA-00604: error occurred at recursive SQL level 1
ORA-01555: snapshot too old: rollback segment number 20 with name "_SYSSMU20$" too small
通过分别dump上述block，我们发现file 1 block 122 有点小问题，如下：
seg/obj: 0x12 csc: 0xd6c.1abf4d18 itc: 1 flg: - typ: 1 - DATA
fsl: 0 fnx: 0x0 ver: 0x01
Itl Xid Uba Flag Lck Scn/Fsc
0x01 0x003f.02e.001cc47d 0x01c012f4.8562.29 --U- 1 fsc 0x0000.1abf4d19
data_block_dump,data header at 0x8b3da44
===============
tsiz: 0x1fb8
hsiz: 0xea
pbl: 0x08b3da44
bdba: 0x0040007a
76543210
flag=--------
ntab=1
nrow=108
通过脚本将该block copy到文件系统，bbed进行修改之后，再copy回asm diskgroup。
接着再次进行scn的推进，可以很顺利打开数据库。
这里需要注意的是，虽然打开了数据库，但是后面还有很多善后处理工作，比如我们dbv发现undo有坏块，那么就需要重建undo；同时检查alert log是否伴随其他的错误。
其次，对于强制open的数据库，我们建议通过mos脚本检查下数据字典是否存在异常；如果数据字典有明显异常，那么通常是需要通过逻辑导出来重建数据库的；否则一般不需要重建库。
对于是否需要重建库，我认为没有定论，安全起见是通常是建议重建数据库；或者数据库很小的时候也可以考虑重建；否则通过检查数据库告警，或者数据库运行一段时间没有其他异常，那么完全可以不重建数据库。